Fragile X syndrome (FXS), caused by the inherited loss of the Fragile X Mental Retardation Protein (FMRP), is the most common form of inherited intellectual disability and the leading monogenetic cause of autism. FMRP binds to many target mRNAs encoding proteins that play key roles at the synapse. FMRP has been shown to repress translation of many target mRNAs, and a few mechanisms have been proposed. FMRP interactions with microRNAs have been shown to play a role in translational control but the molecular mechanisms are not well understood. FMRP mediated repression of translation is reversibly regulated and dependent on the phosphorylation status of FMRP. FMRP has also been shown to be ubiquitinated in response to glutamate receptor stimulation, providing a potential mechanism to dynamically remove translational repression. It remains unclear whether any of the above mechanisms occur locally within dendrites to regulate local translation important for protein synthesis dependent synaptic plasticity. It is likely that some or all of these mechanisms are inter-related but critical details are lacking to understand FMRP mediated translational control and its reversibility in response to receptor signaling. A critical gap is lack of a unifying model for FMRP mediated repression and its reversible regulation at the synapse. We hypothesize that FMRP ubiquitination and UPS-mediated degradation in response to receptor stimulation provides a unifying mechanism to remove translational repression and regulate local protein synthesis at the synapse. The specific role of the E3 ligase Cdh1-APC in FMRP mediated regulation of local protein synthesis will be investigated. To elucidate the local functions of these mechanisms within dendrites and spines, we will continue to develop and apply fluorescent reporters and single molecule imaging of live cultured hippocampal neurons. Using dissociated and organotypic slice cultures as model systems, the role of UPS mediated FMRP degradation, as a local translational switch, to regulate spine morphology, synapse development and plasticity will be investigated. We will analyze the role of FMRP mutants that are resistant to ubiquitination or unable to bind Cdh1-APC to modulate or rescue FXS-associated impairments in dendritic spine development, synapse function and plasticity. Aim 1 will test the hypothesis that FMRP dephosphorylation, ubiquitination by Cdh1-APC and UPS- mediated degradation are components of a dynamic molecular switch to regulate local mRNA translation that functions in control of dendritic spine morphology, synapse development and plasticity. Aim 2 will test the hypothesis that FMRP ubiquitination and UPS-mediated degradation provides a mechanism to regulate targeting of RISC/microRNAs. This research is expected to uncover a novel role for Cdh1-APC and FMRP ubiquitination in regulation of microRNAs and local protein synthesis. The development of disease mechanism based therapeutic strategies for FXS will benefit from this in depth understanding of the mechanism and function of FMRP mediated control of local mRNA translation at synapses.